Aggregates of a hydrazono-sulfonamide adduct as picric acid sensors

Abstract

A novel hydrazono-sulfonamide adduct (AVM) was designed and synthesized. Due to the inhibition of C=N isomerization with a higher water content, the adduct shows remarkable aggregation induced emission enhancement (AIEE) properties in a THF : water solvent system. The formation of nanoaggregates was confirmed by transmission electron microscopy (TEM) analysis. Theoretical DFT calculations supported the observed photophysical changes. These aggregates of AVM act as selective and sensitive sensors for picric acid via a fluorescence quenching mechanism. The efficiency of the quenching process was calculated using the Stern–Volmer equation. The detection limit was found to be 80 nM. In addition, contact mode detection using fluorescent test strips was developed to demonstrate the solid phase sensing of picric acid.

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Introduction

Owing to its strong acidity and high water solubility,¹ picric acid (PA) is considered as a threat to human beings and a great hazard to the environment.² Consequently, for the past few years, the detection of picric acid has been a field of enormous interest to researchers.³ Due to the explosive nature of PA and the increasing terrorist activities all over the world, it is necessary to develop sensitive and selective fluorescent probes for the detection of PA. In spite of the more explosive nature of PA compared to other nitro aromatic compounds (NACs), the development of sensors for the detection of PA has received less attention than the development of sensors for other NACs. Several methods and materials such as metal complexes,⁴ ionic liquids,⁵ organic gels,⁶ polymers,⁷ metal–organic frameworks,⁸ and quantum dots⁹ have been reported and are useful for the detection of NACs. As fluorescent signalling involves simple instrumentation, high sensitivity, selectivity, fast response and importantly low cost, the merits of fluorescent based detection of NACs among various other methods have been discussed in a good number of publications.¹⁰ Owing to the impact of PA, Kumar et al.,¹¹ Mukherjee et al.,¹² Tang et al.,¹³ and others¹⁴ have made significant contributions towards the preparation of various ensembles for the detection of PA.

Aggregation induced emission, which is exactly opposite to concentration quenching or aggregation caused quenching, was first reported by Tang et al. in 2001.¹⁵ In the solution state, certain molecules are weakly emissive but emit strongly in the condensed phase or the aggregated state.¹⁶ In the aggregation phenomenon, molecules tend to form highly emissive nanoparticles, which are effectively utilized as sensors for various analytes.¹⁷ The restriction of intramolecular motion has been proven to be a primary reason for the AIE of many luminogens. In addition, many of the reported probes for PA sensors were big molecules with high molecular weights, needed costly raw materials for preparation, and involved multistep syntheses. Therefore, cost effective, highly-selective and sensitive small molecule based sensors for PA are still in demand. In our recent publication,¹⁸ we had disclosed a novel method for the preparation of a hydrazono-sulfonamide adduct using a heterogeneous catalyst, which shows remarkable aggregation induced emission enhancement (AIEE) properties. We said that the inhibition of the C=N isomerization with a higher water content accounted for the observed AIEE character of the adduct. In continuation of our previous work, we herein report a novel hydrazono-sulfonamide adduct AVM as a fluorescent chemosensor for PA, which employs easy synthetic operation and shows superior selectivity towards PA over other NACs with a high sensitivity and quenching constant and a low detection limit. To the best of our knowledge, a PA sensor using the C=N isomerization concept is unprecedented.

Experimental section

Materials and methods

Tosyl azide¹⁹ was prepared by adopting the method reported in the literature. Preparation and characterization data for Cu(BTC) MOF are presented in our reported protocol.¹⁸ Unless otherwise stated, all solvents and chemicals were obtained from commercial sources and used without further purification. Analytical thin layer chromatography (TLC) was performed on pre-coated silica gel-G plates (Merck) using a mixture of petroleum ether (60–80 °C) and ethyl acetate (7 : 3) as the eluent. The ¹H and ¹³C NMR spectra were recorded on a Bruker (Advance) 300 MHz instrument using TMS as an internal standard and CDCl₃ as a solvent. Chemical shifts are expressed in parts per million (ppm) and the coupling constants (J values) are expressed in hertz (Hz). The following abbreviations are used to indicate spin multiplicities: s (singlet), m (multiplet). Elemental analyses were carried out using a Perkin-Elmer 2400 series II analyzer. Melting points were determined using open capillaries and were uncorrected. Absorption measurements were carried out in an Agilent single beam UV-Diode Array spectrophotometer. Fluorescence spectra were recorded using an Agilent Cary Eclipse Fluorescence spectrophotometer. Electrospray ionization mass spectrometry (ESI-MS) was recorded using a LCQ Fleet, Thermo Fisher Instruments Limited, US. The morphological characterization of the nanoaggregates was determined using a JEOL JEM-2010 Transmission Electron Microscope. Single crystal XRD analysis was performed using a 3-circle Bruker Apex X-ray diffractometer. The slit width was 5 nm for both excitation and emission. HPLC grade solvents were used for photophysical measurements.

Preparation of aggregates

A stock solution of AVM was prepared in THF (10⁻⁴ M). An aliquot (1 mL) of this stock solution was transferred to a volumetric flask. An appropriate amount of THF and water was added to the flask under vigorous stirring to get the 10⁻⁵ M THF : water mixtures with water fractions of 0–90%. The spectral analyses of the resultant mixtures were measured immediately.

Caution! The nitroaromatic compounds used in this study, specially TNT and picric acid, are very powerful explosives. They must be handled with care and also in very small quantities.

Synthesis of probe AVM

Adduct AVM has been synthesized by following our recently reported protocol.¹⁸ To the stirring mixture of N,N-dimethylamino benzaldehyde 1 (1 mmol) and phenylhydrazine 3 (1 mmol) in DCM (3 mL) phenyl acetylene 2 (1 mmol), tosyl azide 4 (1 mmol) and activated Cu(BTC) MOF (1 mol%) were added. To the above mixture, triethylamine (1.1 mmol) was added slowly. The whole reaction mixture was agitated for 10 min at room temperature and then filtered to separate the catalyst from the reaction mixture. To the filtrate petroleum ether and an ethyl acetate mixture (1 : 1, 20 mL) were added and the resulting mixture was stirred for 10 min. About 75% of the solvent mixture was distilled off from the filtrate under vacuum at 70 °C. The above crude product was cooled to 0 to 5 °C and triturated immediately for 5–8 min to afford a light yellow solid which was collected by filtration. The recovered catalyst was thoroughly washed with DCM and air dried for 10 min before using it for the next reaction. Isolated yield 80% (1.4 g); light yellow solid; mp 191–193 °C; ¹H NMR (300 MHz, CDCl₃): δ 7.54–7.44 (m, 7H), 7.35–7.25 (m, 4H), 7.20–7.16 (m, 2H), 7.07 (t, J = 7.5 Hz, 4H), 6.59 (d, J = 9.0 Hz, 2H), 4.95 (s, 2H), 2.98 (s, 6H), 2.33 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 167.5, 151.8, 146.3, 141.6, 140.8, 136.6, 136.0, 129.9, 129.1, 129.1, 128.9, 128.7, 128.7, 128.4, 126.3, 125.9, 121.1, 111.6, 40.0, 36.9, 21.2; MS (ESI) m/z [M + H]⁺: 511.2; anal. calcd for: C₃₀H₃₀N₄O₂S: C, 70.56; H, 5.92; N, 10.97%. Found C, 70.60; H, 5.87; N, 10.92%.

Results and discussion

The synthetic route for the preparation of adduct AVM is outlined in Scheme 1. The adduct was fully characterized using NMR, ESI-MS and elemental analysis. Single crystals of AVM were obtained by the slow evaporation of a solution of AVM in ethyl acetate and pet ether solvent mixtures (Fig. 1 and CCDC 1404294†). X-ray diffraction analysis revealed that the adduct is crystallised in the monoclinic crystal system with the P2₁/c space group. The crystallographic parameters of the adduct are provided in Table 1. From the crystal structure, it was found that both the imines are in E configurations. The adduct is highly soluble in common organic solvents such as THF, chloroform, dichloromethane, etc. and insoluble in water. We envisaged that AVM could be weakly emissive because of the active C=N isomerization, which can be arrested in the aggregated state.^18,20

Scheme 1 Synthesis of adduct AVM.

Fig. 1 ORTEP diagram of AVM.

Table 1 Crystal data and structure refinement parameters of AVM

Parameters	AVM
a R1 = ∑|Fo − Fc|/∑|Fo|.b wR2 = {∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]}^1/2.
Formula	C30H30N4O2S
Formula weight	510.2
Crystal system	Monoclinic
Space group	P21/c
a/Å	13.4218(17)
b/Å	10.5641(11)
c/Å	19.169(3)
α/°	90
β/°	106.172(7)
γ/°	90
V/Å^3	2610.4(6)
Z	4
Temperature (K)	100
D, g cm^−3	1.308
μ (Mo-Kα) mm^−1	0.160
F(000)	1086
Size (mm)	0.08 × 0.15 × 0.25
θ range (°)	1.6–25.0
Reflection collected	19 348
R1^a [I > 2σ(I)]	0.0671
wR2^b [I > 2σ(I)]	0.1817
Goodness-of-fit	1.10

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We screened the photophysical properties of adduct AVM using UV-Vis and fluorescence spectroscopy techniques in THF and THF : water mixtures by maintaining the concentration at 1 × 10⁻⁵ M. The UV-Vis spectrum (ESI†) of adduct AVM in THF shows an absorption maximum at 354 nm. When water (90% volume fraction) was added to the THF solution of AVM, the absorption band was slightly red shifted to 371 nm with the appearance of a leveling-off long wavelength tail due to the Mei scattering effect, which confirms the formation of nanoparticles.²¹ The photoluminescence (PL) spectra of AVM in THF and THF : water mixtures are shown in Fig. 2. As depicted in Fig. 2, AVM is weakly emissive when it is dissolved in THF and exhibits an emission band at 435 nm. When 10 to 50% of water was added, the PL intensity gradually increased with a shift in the wavelength from 434 to 460 nm. At the same time, the PL intensity of the samples with water fractions of 60, 70 and 80% decreased. In contrast, the PL intensity of the sample with a 90% water fraction reached a maximum and the emission maximum was located at 438 nm. A similar kind of spectral change was frequently observed for compounds with AIEE properties, but the reason for it remains unclear.²² We assumed that the inhibition of C=N isomerization could be the primary reason for the observed spectral changes. Because of the C=N isomerization of AVM was weakly emissive in THF solution, when water was added the C=N isomerization gets blocked causing the suppression of non-radiative decay from the excited state.¹⁸ After the addition of water, the solute molecules can aggregate into various kinds of nanoparticle suspensions: crystal particles and amorphous particles.^22f The former leads to an enhancement in the PL intensity, while the latter leads to a reduction in intensity. Thus, the measured overall PL intensity data depends on the combined actions of the two kinds of nanoparticles. However, it is difficult to control the formation of nanoparticles when the water content is high. Thus, the measured PL intensity often shows no regularity in samples with a high water content. Transmission electron microscopy (TEM) further confirmed the formation of spherical nanoparticles (Fig. 3a).

Fig. 2 PL spectra of AVM (1 × 10⁻⁵ M) in THF : water mixtures.

Fig. 3 TEM image of the nano-aggregates for a 90% water fraction (a), color change between samples with a 0 and 90% water fraction under UV irradiation (b), solid state emission in day light (c) and UV light, 365 nm (d).

The ground state geometry of AVM was optimized using a density functional theory (DFT) method using B3LYP/6-31G basis sets. The DFT calculations were carried out using the Gaussian 09 program.²³ The ground state optimized geometries and absorption behaviours of the corresponding transitions of the compounds were obtained from time dependent (TD) DFT using the above mentioned functional and basis set. The DFT optimized structure indicates a similar E configuration for both the imine groups in the gas phase which is the same as the crystal structure of the adduct (Fig. 4b). The frontier molecular orbital diagram (Fig. 4a) shows that the highest occupied molecular orbital (HOMO) is mainly delocalized on the N,N-dimethyphenyl moiety indicating the n–π* transition. The lowest unoccupied molecular orbital (LUMO) is delocalized on the N,N-dimethyphenyl and sulfonyl groups. The calculated energy gap between the HOMO and LUMO is 3.9 eV.

Fig. 4 HOMO and LUMO energy levels of AVM calculated using TD-DFT/B3LYP/6-31G basic sets (a) and the optimized geometry (b).

Picric acid detection

The highly emissive nature of AVM in the condensed phase prompted us to explore the possibility of its application as a sensor. In general, electron deficient NACs tend to interact with electron rich molecules and electron donating groups such as free amines and N,N-dialkyl or aryl amino groups. Based on this concept several fluorescent sensors have been developed for NACs.²⁴ We hypothesized that AVM could act as a probe for NACs as it possesses a N,N-dimethyl amino group. To confirm our hypothesis, we commenced our work using PA as a model NAC. We used nanoaggregates from 50% and 90% water mixtures as picric acid probes along with the pure THF solution of AVM for comparison.

Emission bands of the pure THF solution of AVM, and the nanoaggregates of the samples with 50% and 90% water fractions were located at 434, 460, and 438 nm when excited at the corresponding absorption maxima. Fluorescence spectroscopic titrations of the nanoaggregates and the pure THF solution of AVM with picric acid revealed that the PL intensity of the corresponding solution decreased with the incremental addition of picric acid (Fig. 5). The highly emissive nature of the nanoaggregates of the samples with 50% and 90% water fractions was completely quenched with 30 equivalents of picric acid, without any appreciable shift in the wavelength. However, the pure THF solution of AVM needed 50 equivalents of picric acid for complete quenching which indicates the utility of the nanoaggregates towards the sensing of PA compared to the pure THF solution. During the addition of PA to the probe, the colorless solution rapidly turned yellow (ESI†).

Fig. 5 PL spectral changes of AVM (1 × 10⁻⁵ M) containing different concentrations of PA in neat THF solution (a) and THF : water mixtures 50 : 50 (b), and 10 : 90 (c). Stern–Volmer plot (d).

The data from the fluorescence titrations were used to calculate the quenching constant using the Stern–Volmer equation,

I₀/I = 1 + K_SV[Q]

where I₀ and I are the PL intensities before and after the addition of PA, [Q] is the quencher concentration and K_SV is the Stern–Volmer or quenching constant (Fig. 5d).

The Stern–Volmer plot (I₀/I vs. PA concentration) is presented in Fig. 5d. The Stern–Volmer constant of the quenching process was found to be 1.0 × 10⁵ M⁻¹ which is comparable to the reported values.^11h,12a,25 A higher quenching constant indicates an effective interaction between the probe and PA, which is further supported by the spectral overlap between the absorption spectrum of PA and the emission spectrum of the probe in the wavelength region of 381–491 nm (ESI†). Linearity at low concentrations of PA in the Stern–Volmer plot (ESI†) indicates the involvement of a static quenching mechanism. However, at higher concentrations of PA, the plot bent upward due to a super amplified quenching effect.²⁶ Nanoaggregates of the sample with a 50% water fraction show a higher sensing performance than those of the 90% water fraction, which may due the polymer packing in the former being looser than in the latter and the looser packing allows voids to interact with more PA molecules.^13c

The turn-off mechanism (Fig. 6) may be explained by the electron transfer and/or energy transfer mechanism^11a,27,28 between the picrate ion and the AVM adduct. The proton NMR spectral studies show the presence of PA, and shift the sensor peak positions downfield (Fig. 7), which further confirms the electrostatic interaction between the sensor and PA.^14c The aromatic protons near to the –NMe₂ group were shifted downfield, thus confirming that the –NMe₂ group is the receptor site. In addition to PA, we screened other NACs (2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), 1,4-dinitrobenzene (DNB), 4-nitrobenzene (NB), 1,4-dinitrophenol (DNP), 4-nitrophenol (NP), 4-nitrobenzoic acid (NBA), and benzoquinone (BQ)) with the probe (Fig. 8). Noticeably, the selectivity is very high toward PA compared to other NACs. Nanoaggregates of AVM offer more diffusion channels for the exciton to migrate, allowing them to be more quickly annihilated by picric acid.²⁹

Fig. 6 Sensing mechanism.

Fig. 7 ¹H NMR spectra of AVM (green) and its PA complex (black) in CDCl₃.

Fig. 8 Relative fluorescence quenching of the nanoaggregates of AVM upon addition of various NACs (30 eq.) in a THF : water (10 : 90) mixture.

The detection limit³⁰ was calculated using the equation 3σ/slope, where σ is the standard deviation. The detection limit was found to be 80 nM, which is almost comparable to the reported values.^14i,31 To explain the merits of this work, we demonstrated the solid phase detection of PA using test strips. Test strips were prepared using TLC plates coated by dipping into a solution of the AVM adduct. The coated strips were dried under vacuum and then used for the detection of PA. The emissive nature of the test strips was quenched when they came into contact with PA. The quenching confirms the solid phase interaction between the probe and PA (Fig. 9).

Fig. 9 Test strip based detection of PA using UV light (365 nm): blank TLC plate (a), after coating with AVM (b) and upon interaction with picric acid (c).

Conclusion

We have designed and synthesized a novel hydrazono-sulfonamide (AVM) adduct using a heterogeneous Cu(BTC) MOF catalyst in good yield. An aggregation study in a THF : water system reveals that C=N isomerization is the main cause for the observed spectral changes. Theoretical DFT calculations further confirmed this fact. Formation of nanoaggregates was confirmed using TEM analysis. Nanoaggregates of the AVM adduct showed high sensitivity and selectivity towards PA compared to other NACs. Furthermore, a low detection limit and high Stern–Volmer constant shows the efficiency of the quenching process. In addition, it was demonstrated that PA can be detected using test strips made from TLC plates.

Acknowledgements

The authors thank DST and UGC for financial assistance and DST-IRHPA for funding the purchase of a higher resolution NMR spectrometer. VM expresses special gratitude to DST-MRP (Reg. No. SR/FT/CS-63/2010) for research fellowship.

References

1. H. Muthurajan, R. Sivabalan, M. B. Talawar and S. N. Asthana, J. Hazard. Mater., 2004, 112, 17.
2. (a) V. Pimienta, R. Etchenique and T. Buhse, J. Phys. Chem. A, 2001, 105, 10037; (b) J. Shen, J. Zhang, Y. Zuo, L. Wang, X. Sun, J. Li, W. Han and R. He, J. Hazard. Mater., 2009, 163, 1199; (c) G. Anderson, J. D. Lamar and P. T. Charles, Environ. Sci. Technol., 2007, 41, 2888–2893; (d) J. F. Wyman, M. P. Serve, D. W. Hobson, L. H. Lee and D. E. J. Uddin, J. Toxicol. Environ. Health, Part A, 1992, 37, 313–327.
3. (a) E. M. Nolan and S. J. Lippard, Chem. Rev., 2008, 108, 3443–3480; (b) M. E. Germain and M. J. Knapp, Chem. Soc. Rev., 2009, 38, 2543–2555; (c) Y. Salinas, R. Martínez-Máñez, M. D. Marcos, F. Sancenón, A. M. Costero, M. Parra and S. Gil, Chem. Soc. Rev., 2012, 41, 1261–1296.
4. (a) G. G. Shan, H. B. Li, H. Z. Sun, D.-X. Zhu, H.-T. Cao and Z.-M. Su, J. Mater. Chem. C, 2013, 1, 1440–1449; (b) V. Sathish, A. Ramdass, Z.-Z. Lu, M. Velayudham, P. Thanasekaran, K.-L. Lu and S. Rajagopal, J. Phys. Chem. B, 2013, 117, 14358–14366.
5. S. Shaligram, P. P. Wadgaonkar and U. K. Kharul, J. Mater. Chem. A, 2014, 2, 13983–21398.
6. (a) B. G. Bag, G. C. Maity and S. K. Dinda, Org. Lett., 2006, 8, 5457–5460; (b) K. K. Kartha, A. Sandeep, V. K. Praveen and A. Ajayaghosh, Chem. Rec., 2015, 15, 252–265.
7. (a) H. Sohn, M. J. Sailor, D. Magde and W. C. Trogler, J. Am. Chem. Soc., 2003, 125, 3821–3830; (b) S. J. Toal and W. C. Trogler, J. Mater. Chem., 2006, 16, 2871–2883; (c) T.-P. Huynh, M. Sosnowska, J. W. Sobczak, B. K. C. Chandra, V. N. Nesterov, F. D’Souza and W. Kutner, Anal. Chem., 2013, 85, 8361–8368; (d) W. Wu, S. Ye, H. L. Xiao, Y. Fu, Q. Huang, G. Yu, Y. Liu, J. Qin, Q. Lia and Z. Li, J. Mater. Chem., 2012, 22, 6374–6382; (e) I.-H. Park, R. Medishetty, J.-Y. Kim, S. S. Lee and J. J. Vittal, Angew. Chem., Int. Ed., 2014, 53, 5591–5595.
8. (a) X. Jiang, Y. Liu, P. Wu, L. Wang, Q. Wang, G. Zhu, X.-L. Li and J. Wang, RSC Adv., 2014, 4, 47357–47360; (b) S. Sanda, S. Parshamoni, S. B. Biswas and S. Konar, Chem. Commun., 2015, 51, 6576–6579.
9. (a) R. Freeman, T. Finder, L. Bahshi, R. Gill and I. Willner, Adv. Mater., 2012, 48, 6416–6421; (b) B. Liu, C. Tong, L. Feng, C. Wang, H. Yao and C. Lü, Chem.–Eur. J., 2014, 20, 2132–2137.
10. (a) D. S. Moore, Rev. Sci. Instrum., 2004, 75, 2499; (b) E. S. Forzani, D. Lu, M. J. Leright, A. D. Aguilar, F. Tsow, R. A. Iglesias, Q. Zhang, J. Lu, J. Li and N. Tao, J. Am. Chem. Soc., 2009, 131, 1390–1391; (c) M. Riskin, R. Tel-Vered, T. Bourenko, E. Granot and I. Willner, J. Am. Chem. Soc., 2008, 130, 9726–9733; (d) R. Hodyss and J. L. Beauchamp, Anal. Chem., 2005, 77, 3607–3610; (e) C.-Y. Lai, B. G. Trewyn, D. M. Jeftinija, K. Jeftinija, S. Xu, S. Jeftinija and V. S.-Y. Lin, J. Am. Chem. Soc., 2003, 125, 4451–4459.
11. (a) V. Vij, V. Bhalla and M. Kumar, ACS Appl. Mater. Interfaces, 2013, 5, 5373–5380; (b) V. Bhalla, S. Kaur, V. Vij and M. Kumar, Inorg. Chem., 2013, 52, 4860–4865; (c) M. Kumar, S. I. Reja and V. Bhalla, Org. Lett., 2012, 14, 6084–6087; (d) V. Bhalla, A. Gupta and M. Kumar, Org. Lett., 2012, 14, 3112–3115; (e) V. Bhalla, H. Arora, H. Singh and M. Kumar, Dalton Trans., 2013, 42, 969–974; (f) S. Kaur, V. Bhalla, V. Vij and M. Kumar, J. Mater. Chem. C, 2014, 2, 3936–3941; (g) S. Kaur, A. Gupta, V. Bhalla and M. Kumar, J. Mater. Chem. C, 2014, 2, 7356–7363; (h) S. Pramanik, V. Bhalla and M. Kumar, Anal. Chim. Acta, 2013, 793, 99–106.
12. (a) S. Shanmugaraju, H. Jadhav, Y. P. Patil and P. S. Mukherjee, Inorg. Chem., 2012, 51, 13072–13074; (b) S. Shanmugaraju, S. A. Joshi and P. S. Mukherjee, Inorg. Chem., 2011, 50, 11736–11745; (c) A. Chowdhury and P. S. Mukherjee, J. Org. Chem., 2015, 80, 4064–4075; (d) B. Roy, A. K. Bar, B. Gole and P. S. Mukherjee, J. Org. Chem., 2013, 78, 1306–1310; (e) K. Acharyya and P. S. Mukherjee, Chem. Commun., 2014, 50, 15788–15791; (f) V. Vajpayee, H. Kim, A. Mishra, P. S. Mukherjee, P. J. Stang, M. H. Lee, H. K. Kim and K.-W. Chi, Dalton Trans., 2011, 40, 3112–3115; (g) A. K. Bar, S. Shanmugaraju, K.-W. Chi and P. S. Mukherjee, Dalton Trans., 2011, 40, 2257–2267; (h) K. Acharyya and P. S. Mukherjee, Chem.–Eur. J., 2015, 21, 6823–6831; (i) S. Shanmugaraju, Chem.–Eur. J., 2015, 21, 6656–6666; (j) B. Gole, S. Shanmugaraju, A. K. Bar and P. S. Mukherjee, Chem. Commun., 2011, 47, 10046–10048; (k) B. Gole, W. Song, M. Lackinger and P. S. Mukherjee, Chem.–Eur. J., 2014, 20, 13662–13680; (l) B. Gole, A. K. Bar and P. S. Mukherjee, Chem.–Eur. J., 2014, 20, 2276–2291.
13. (a) H. Li, H. Wu, E. Zhao, J. Li, J. Z. Sun, A. Qin and B. Z. Tang, Macromolecules, 2013, 46, 3907–3914; (b) J. Liu, Y. Zhong, J. W. Y. Lam, P. Lu, Y. Hong, Y. Yu, Y. Yu, M. Faisal, H. H. Y. Sung, I. D. Williams, K. S. Wong and B. Z Tang, Macromolecules, 2010, 43, 4921–4936; (c) R. Hu, J. L. Maldonado, M. Rodriguez, C. Deng, C. K. W. Jim, J. W. Y. Lam, M. M. F. Yuen, G. R. Ortiz and B. Z. Tang, J. Mater. Chem., 2012, 22, 232–240; (d) Y. Zhang, G. Chen, Y. Lin, L. Zhao, W. Z. Yuan, P. Lu, C. K. W. Jim, Y. Zhang and B. Z. Tang, Polym. Chem., 2015, 6, 97–105; (e) P. Lu, J. W. Y. Lam, J. Liu, C. K. W. Jim, W. Yuan, N. Xie, Y. Zhong, Q. Hu, K. S. Wong, K. K. L. Cheuk and B. Z. Tang, Macromol. Rapid Commun., 2010, 31, 834–839; (f) C. Y. K. Chan, Z. Zhao, J. W. Y. Lam, J. Liu, S. Chen, P. Lu, F. Mahtab, X. Chen, H. H. Y. Sung, H. S. Kwok, Y. Ma, I. D. Williams, K. S. Wong and B. Z. Tang, Adv. Funct. Mater., 2012, 22, 378–389.
14. (a) Y. Peng, A. J. Zhang, M. Dong and Y.-W. Wang, Chem. Commun., 2011, 47, 4505–4507; (b) Y. Xu, B. Li, W. Li, J. Zhao, S. Sun and Y. Pang, Chem. Commun., 2013, 49, 4764–4766; (c) P. Vishnoi, S. Sen, G. N. Patwari and R. Murugavel, New J. Chem., 2015, 39, 886–892; (d) L. Wenfeng, M. H. Chang and L. Ziqiang, RSC Adv., 2014, 4, 39351–39358; (e) R. Kumar, S. Sandhu, P. Singh, G. Hundal, M. S. Hundal and S. Kumar, Asian J. Org. Chem., 2014, 3, 805–813; (f) H.-T. Feng and Y.-S. Zheng, Chem.–Eur. J., 2014, 20, 195–201; (g) J. Ye, X. Wang, R. F. Bogale, L. Zhao, H. Cheng, W. Gong, J. Zhao and G. Ning, Sens. Actuators, B, 2015, 210, 566–573; (h) R. Chopra, P. Kaur and K. Singh, Anal. Chim. Acta, 2015, 864, 55–63; (i) J.-F. Xiong, J.-X. Li, G.-Z. Mo, J.-P. Huo, J.-Y. Liu, X.-Y. Chen and Z.-Y. Wang, J. Org. Chem., 2014, 79, 11619–11630; (j) X. He, P. Zhang, J.-B. Lin, H. V. Huynh, S. E. Navarro Muñoz, C.-C. Ling and T. Baumgartner, Org. Lett., 2013, 15, 5322–5325.
15. J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem. Commun., 2001, 1740–1741.
16. (a) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2011, 40, 5361–5388; (b) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun., 2009, 4332–4353; (c) J. Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang and B. Z. Tang, Adv. Mater., 2014, 26, 5429–5479; (d) Z. Zhao, J. W. Y. Lam and B. Z Tang, Curr. Org. Chem., 2010, 14, 2109–2132.
17. (a) T. Sanji, K. Shiraishi, M. Nakamura and M. Tanaka, Chem.–Asian. J., 2010, 5, 817–824; (b) M. Wang, D. Zhang, G. Zhang, Y. Tang, S. Wang and D. Zhu, Anal. Chem., 2008, 80, 6443–6448; (c) Q. Zhao, K. Li, S. Chen, A. Qin, D. Ding, S. Zhang, Y. Liu, B. Liu, J. Z. Sun and B. Z. Tang, J. Mater. Chem., 2012, 22, 15128–15135; (d) M. Wang, G. Zhang, D. Zhang, D. Zhu and B. Z. Tang, J. Mater. Chem., 2010, 20, 1858–1867; (e) E. Wang, E. Zhao, Y. Hong, J. W. Y. Lam and B. Z. Tang, J. Mater. Chem. B, 2014, 2, 2013–2019.
18. V. Mahendran and S. Shanmugam, RSC Adv., 2015, 5, 20003–20010.
19. H. Lu, V. Subbarayan, J. Tao and X. P. Zhang, Organometallics, 2010, 29, 389–393.
20. K. Namitharan and K. Pitchumani, Org. Biomol. Chem., 2012, 10, 2937–2941.
21. B. Z. Tang, Y. Geng, J. W. Y. Lam, B. Li, X. Jing, X. Wang, F. Wang, A. B. Pakhomov and X. X. Zhang, Chem. Mater., 1999, 11, 1581–1589.
22. (a) X. Zhang, Z. Chi, B. Xu, C. Chen, X. Zhou, Y. Zhang, S. Liu and J. Xu, J. Mater. Chem., 2012, 22, 18505–18513; (b) G. Zhang, A. Ding, Y. Zhang, L. Yang, L. Kong, X. Zhang, X. Tao, Y. Tian and J. Yang, Sens. Actuators, B, 2014, 202, 209–216; (c) A. Ding, L. Yang, Y. Zhang, G. Zhang, L. Kong, X. Zhang, Y. Tian, X. Tao and J. Yang, Chem.–Eur. J., 2014, 20, 12215–12222; (d) H. Li, Z. Chi, B. Xu, X. Zhang, Z. Yang, X. Li, S. Liu, Y. Zhang and J. Xu, J. Mater. Chem., 2010, 20, 6103–6110; (e) S. C. Dong, Z. Li and J. G. Qin, J. Phys. Chem. B, 2009, 113, 434–441; (f) X. Zhang, Z. Yang, Z. Chi, M. Chen, B. Xu, C. Wang, S. Liu, Y. Zhang and J. Xu, J. Mater. Chem., 2010, 20, 292–298.
23. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009.
24. (a) J. M. T. Lin, X. Pan and W. Wang, Chem. Mater., 2014, 26, 4221–4229; (b) B. Xu, X. Wu, H. Li, H. Tong and L. Wang, Macromolecules, 2011, 44, 5089–5092; (c) Y. Xu, B. Li, W. Li, J. Zhao, S. Sun and Y. Pang, Chem. Commun., 2013, 49, 4764–4766; (d) W. Wei, R. Lu, S. Tang and X. Liu, J. Mater. Chem. A, 2015, 3, 4604–4611; (e) K. D. Prasad and T. N. G. Row, RSC Adv., 2014, 4, 45306–54531; (f) G. Sivaraman, B. Vidya and D. Chellappa, RSC Adv., 2014, 4, 30828–30831; (g) R. Chopra, P. Kaur and K. Singh, Anal. Chim. Acta, 2015, 864, 55–63.
25. (a) J. Wang, J. Mei, W. Yuan, P. Lu, A. Qin, J. Sun, Y. Ma and B. Z. Tang, J. Mater. Chem., 2011, 21, 4056–4059; (b) D. Li, J. Liu, R. T. K. Kwok, Z. Liang, B. Z. Tang and J. Yu, Chem. Commun., 2012, 48, 7167–7169.
26. N. Venkatramaiah, S. Kumar and S. Patil, Chem.–Eur. J., 2012, 18, 14745–14751.
27. T. Liu, L. Ding, G. He, Y. Yang, W. Wang and Y. Fang, ACS Appl. Mater. Interfaces, 2011, 3, 1245–1253.
28. J. Ma, T. Lin, X. Pan and W. Wang, Chem. Mater., 2014, 26, 4221–4229.
29. J. Wang, J. Mei, W. Yuan, P. Lu, A. Qin, J. Sun, Y. Ma and B. Z. Tang, J. Mater. Chem., 2011, 21, 4056–4059.
30. (a) B. Yu, J. Ma, Y. Zhang, G. Zou and Q. Zhang, RSC Adv., 2015, 5, 29262–29265; (b) N. Venkatesan, V. Singh, P. Rajakumar and A. K. Mishra, RSC Adv., 2014, 4, 53484–53489.
31. Y. Xu, B. Li, W. Li, J. Zhao, S. Sun and Y. Pang, Chem. Commun., 2013, 49, 4764–4766.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1404294. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra17359k

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